In the course of processing a photographic element containing an imagewise exposed silver halide emulsion layer reduced silver can be formed either as a direct or inverse function of exposure. At the same time, at least a low level of reduced silver formation also occurs independently of imagewise exposure. The term "fog" is herein employed to indicate the density of the processed photographic element attributable to the latter, usually measured in minimum density areas. In color photography fog is typically observed as image dye density rather than directly as silver density.
Over the years a variety of materials have been introduced into silver halide emulsions to inhibit the formation of fog. Research Disclosure, Vol. 176, December 1978, Item 17643, Section VI, lists the more commonly employed fog inhibiting agents. Research Disclosure is published by Kenneth Mason Publications Limited; Emsworth; Hampshire P010 7DD; England. From Section VI it is apparent that useful fog inhibiting agents are highly diverse in their structural forms, ranging from halide ions (e.g. bromide salts) to inorganic metal salts to specific polymers to selected acyclic organic compounds to specific heterocycles. These useful fog inhibiting agents have been selected from among a plethora of structurally similar, but relatively ineffective compounds. Useful fog inhibiting agents have been largely identified empirically. T. H. James, The Theory of the Photographic Process, 4th Ed., Macmillan, 1977, pp. 393-399, in grouping and suggesting various performance mechanisms for fog inhibiting agents illustrates their diversity.
Gunther et al U.S. Pat. No. 4,576,905, commonly assigned, discloses to be useful as intermediates in synthesizing aromatic tellurazolium salts compounds containing a 1,2,5-oxatellurazinium ring fused with an aromatic ring. The synthesis taught by Gunther et al results in the nitrogen atom of the 1,2,5-oxatellurazinium ring being protonated. Specifically, oxatellurazinium salts satisfying formula (I) ##STR1## are prepared by reacting a compound according to formula (II) ##STR2## with tellurium tetrachloride or tellurium tetrabromide at an elevated temperature,
wherein:
H* is an activated hydrogen atom, PA0 G represents the atoms completing an aromatic nucleus, PA0 R represents an aliphatic or aromatic group comprised of a hydrocarbon moiety optionally linked through a divalent oxy, thio, or carbonyl linkage, an amino group, an amido group, a ureido group, a formamidine disulfide group, or a --C(O)M group, wherein M is chosen to complete an acid, ester, thioester, or salt, and PA0 X represents chloride or bromide or another halogen or a pseudohalogen substituted therefor after preparation. PA0 R represents hydrogen or an alkyl moiety optionally linked through a divalent oxy, thio, or carbonyl linkage, said alkyl moiety containing from 1 to 6 carbon atoms; PA0 Q represents an alkyl, sulfoalkyl, or sulfatoalkyl substituent of from 1 to 6 carbon atoms or a dioxolane ring; and PA0 X represents bromide or chloride. PA0 G represents the atoms completing an aromatic nucleus; PA0 R represents hydrogen, an aliphatic or aromatic group comprised of a hydrocarbon moiety optionally linked through a divalent oxy, thio, or carbonyl linkage, an amino group, an amido group, a ureido group, a formamidine disulfide group, or a --C(O)M group, wherein M is chosen to complete an aldehyde, acid, ester, thioester or salt; PA0 Q represents a quaternizing substituent; PA0 X represents chloride or bromide as initially prepared, but which can be extended to another halogen or a pseudohalogen by subsequent substitution; PA0 X' represents a charge balancing counter ion; and PA0 n represents the integer 0 or 1. PA0 T.sup.1 is independently carbonyl (CO) or sulfonyl (SO.sub.2) in each occurrence; and PA0 m is an integer of from 1 to 3. PA0 L represents a divalent linking group, such as an optionally substituted divalent hydrocarbon group; and PA0 R.sup.5 represents an optionally substituted hydrocarbon residue or an amino group. PA0 G represents the atoms completing a fused aromatic nucleus, PA0 R is an optionally substituted hydrocarbon moiety, PA0 m is zero or 1, PA0 X is halogen or pseudohalogen, PA0 Y is halogen or R--C(O)--O--, PA0 Z is --O-- or --N(R')--, and PA0 R' is an aromatic nucleus. PA0 G represents the atoms completing an aromatic nucleus, PA0 R represents hydrogen, an optionally substituted hydrocarbon moiety, or a --C(O)M group, wherein M is chosen to complete an aldehyde, acid, ester, thioester, or salt, and PA0 R.sup.4 represents a leaving group.
Repeated attempts to prepare a quaternized oxatellurazinium salt by substituting a quaternizing substituent for the hydrogen atom bonded to the nitrogen atom in formula (II) have uniformly met with failure. Thus, Gunther et al teaches protonated oxatellurazinium salts and does not disclose or suggest a feasible method of preparing quaternized oxatellurazinium salts.
Lok et al U.S. Ser. No. 702,545, filed Feb. 19, 1985, titled TELLURIUM SALT FOG INHIBITING AGENTS FOR SILVER HALIDE PHOTOGRAPHY, commonly assigned, discloses the oxatellurazinium salts of Gunther et al to be useful fog inhibiting agents. Lok et al discloses only the method of Gunther et al for preparing the oxatellurazinium salts.